Detection of toxic organophosphoru airborne substances by frustrated multiple internal reflection spectroscopy

ABSTRACT

A thin germanium crystal prism in the form of an isosceles trapezoid or parallelogram whose base surface and opposite parallel surface are coated with platinum about 70 millimicrons thick, the coating having been applied in a vacuum on the order of 10 millimicrons for effective adsorptiveness. Sampled atmosphere is flowed across the base and the opposed surface of the crystal while infrared of wavelength band 9-10 microns is directed through the crystal for multiple internal reflection and the amplitude of infrared that passes through the crystal is monitored.

United States atent 3,515,490 6/1970 Dreyfuset al WH ER IA/ C [72]Inventors Charles N. La Rosa OTHER REFERENCES New y Park "AppliedSpectroscopy", Volume 21, No. l, .lanua Manfred J- s Elizabeth. -J-;Joseph ry/February, 1968, pages 48 and 49 relied on Kalinskl" m La Rosaet al. Page 92 of the detailed program of The Pitt 1 pp 775338 sburghConference on Analytical Chemistry and Appliel Filed Spectroscopy, March6- I0, 1967, Handout [45] Patented June [#1971 .Prager et al., AppliedSpectroscopy", Volume 2, No. 5 {73] Assignee The United Statesol'Amerlca as September/October 1968 pages 451 represented by theSecretary of the Navy Primary Examiner-Ronald L. Wilbert AssistantExaminerF. L. Evans [54] DETECTION OF TOXIC ORGANOPHOSPHORUAt!0rneys-Louis A. Miller, Louis B. Applebaum and Arthur AIRBORNESUBSTANCES BY FRUSTRATED L. Bowers MULTIPLE INTERNAL REFLECTIONSPECTROSCOPY l Clam 7 Drawing Figs ABSTRACT: A thin germanium crystalprism in the form 01 [52] U-S-Cl 356/51, an isosceles trapezoid orparallelo ram whos b s rf c 250/83.3H. 356/ 3 and opposite parallelsurface are coated with platinum about [5]] Int.Cl 21/34 70 millimicronsthick the coating having been in a of Search SI, vacuum on the order ofmillimicrons for effective adgorp. 2 2 IR tiveness. Sampled atmosphereis flowed across the base and the opposed surface of the crystal whileinfrared of [56] References cued wavelength band 9l0 microns is directedthrough the UNITED STATES PATENTS crystal for multiple internalreflection and the amplitude of infrared that passes through the crystalis monitored.

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DETECTION OF TOXIC ORGANOPIIOSPIIORU AIRBORNE SUBSTANCES BY FRUSTRATEDMULTIPLE INTERNAL REFLECTION SPECTROSCOPY The invention described hereinmay be manufactured and used by or for the Government of the UnitedStates of America for governmental purposes without the payment of anyroyalties thereon or therefor.

The atmosphere is monitored for contamination by a variety of suspectedtoxic substances. For example, concentration of sulfur dioxide emittedby the stacks of fossil fuel power plants is now monitored routinely.Similarly, the atmosphere is monitored for toxic organophosphoruscompounds that may be spewed into the atmosphere primarily asinsecticides. Error, poor judgment, unanticipated change in weather,even antisocial intent, can lead to hazardous concentration of suchsubstances in the atmosphere in local areas. Insecticidal phosphoruscompounds are similar in chemical structure to the nerve gases e.g.phosgene though not nearly as toxic.

Spectroscopic methods for examining samples of materials for traces oftoxic organophosphorus substances are known but the methods arecumbersome, are limited to isolated checks as opposed to continuousmonitoring and require trained laboratory personnel. These prior methodscannot provide essentially instantaneous warning of a hazardoussituation.

An object of this invention is to continuously monitor the atmosphere todetect the presence of trace quantities of selected substancesparticularly toxic organophosphorus substances.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

FIGS. l-3 illustrate an optical phenomenon utilized in this invention;

FIG. 4 is a perspective view of a coated multiple internal reflectioncell, in accordance with this invention;

FIG. 5 is an atmosphere monitoring apparatus mounting a cell as in FIG.4 but having parallel sides and FIG. 6 is a sec tion taken on line 6-6of FIG. 5 on a larger scale, and

FIG. 7 is a graphical showing of the relationship of coating thicknessand attenuation response.

FIG. I shows a germanium crystal in the form of an isosceles triangularprism 10 having mirror smooth polished slant faces 11, 12 and base 13. Abeam 14 of energy in the infrared band directed to the slant face 11 at90 is transmitted linearly to the base 13. If the angle of incidence tothe base is greater than the critical angle, substantially all theenergy incident to the prism exits from slant face 12. However, if thebase surface is smeared as in FIG. 2 with an adherent matrix material I5which makes intimate contact with the base surface and which contains adispersed trace of a substance that absorbs energy within that infraredband, an incident beam 14 of infrared energy is attenuated at particularwavelengths compared to beam 14 ofinfrared energy incident to the prismI0 as in FIG. I without a smear on its base whereby the presence of,asuspect contaminant is detected. Presumably the infrared beam penetratesthe smear in reflection toward the other slant face and loses energy oninteracting with the trace of contaminant that absorbs energy within theband of infrared beam 14. Because the infrared beam 14 cannot see thecontaminant substance in the atmosphere directly when the prism isexposed to that atmosphere, a trace of airborne suspect substance iscaptured in the matrix material by any conventional techniques such asforcing a quantity of air through a filter or bubbling a quantity ofatmosphere through a wash liquid and transferring some of the trappedsubstance to the matrix material 15.

The technique of coating the prism base with a matrix containing asubstance to be studied is utilized in spectrophotometry and serves as avaluable tool for the analytical chemist to supplement transmissionabsorption techniques. The band of incident energy beam 14 isessentially flat. Intensity v wavelength of the energy that exits fromthe prism is plotted by the spectrophotometer.

The effectiveness of the phenomenon shown in FIG. 2 is enhanced bymultiple internal reflections as illustrated in FIG. 3. A thin crystalI6, with parallel sides 17 and I8 at 45 to the base carries coating 15on its base and on the parallel surface. Attenuation ofinfrared energy14 at the characteristic absorption frequency or frequencies of thecontaminant substance in coating 15 is repeated at each reflection.

In this invention, a crystal 19, as shown in FIG. 4 preferablygermanium, is coated on its base and on the opposite parallel face witha film 20 of platinum on the order of millimicrons thick applied byvacuum deposition in a vacuum of about 10 millimicrons. In FIGS. 5 and6, the crystal 19 is seated in a pair of opposed holders 22 betweentemperature control means 24 and is located within a black-surfacednonreflective rectangular frame 26 supported in a housing 28. Thesidewalls of the frame 26 are formed with two holes each, through whichextend loosely fitted short lengths of rigid tubing 30. A pneumaticchannel element 32 is penetrated and carried by and sealed to each pairof short lengths of tubing 30 that extend through one sidewall of frame26. Each channel element 32 is an elongate, comparatively yieldablerecessed plastic member backed by a rigid plate 34. Clamping screws 36are threaded into the sidewall of the frame for engaging the backingplates and forcing the pneumatic channel elements against opposite facesof the crystal I9 mounted in the holders. Flexible tubing 38 is attachedto the free end of each of the short rigid tubings 30. Flexible tubings38 extending from corresponding ends of the two pneumatic channelelements are joined by tees 40. A fan or pump not shown for continuouslydrawing a sample of atmosphere delivers the sampled atmosphere throughtubing 42 to one tubing pair for flowing atmosphere sampling across theopposite faces of the crystal 1). The sampled atmosphere is exhaustedthrough the tubing 44 a substantial distance from the intake. A set ofmirrors is arranged to direct infrared energy through the multipleinternal reflection crystal and thence to a spectrophotometer. Eachtemperature control means 24 is a peltier unit for heating or coolingbacked by metal conduit 46 connected to tubings 48 shown only in part inFIG. 5 for circulating water to raise or lower the temperature of thecrystal.

In its simplest form this invention may be used for monitoring fororganophosphorus compounds by passing the infrared energy from thesource through a very narrow band filter that transmits infrared ofapproximately 9.6 micron wavelength, which interacts withorganophosphorus compounds. The energy that passes through the crystalis continuously monitored photoelectrically for rapid attenuation at theoutput from the crystal for triggering an alarm circuit. The responsetime of this arrangement is short, less than a minute, to the presenceof dangerously high concentration of toxic organophosphorus substancesin the atmosphere. The temperature of the crystal is reduced fromambient to about 15C. by the temperature control means 24 and 46 forefficient response to the presence of contaminant. Monitoring iscontinuous and requires no attention. A straightforward amplifier andrelay in the amplifier circuit for coupling and decoupling the alarm andits power supply suffice for the purpose. The contaminant absorbed bythe coated crystal is carried away gradually when the concentration ofthe toxic substance in the sampled atmosphere drops substantially butcan be speeded by operating the temperature control means 24 and 46 toraise the temperature of the crystal substantially above ambienttemperature. A contaminated cell is automatically decontaminated after atime by the continuous flow of sampled atmosphere when the sampledatmosphere no longer carries contaminant, whether or not the celltemperature is elevated. The cell is usable over and over. Instead ofbeing used with an alarm, the invention may be used with aspectrophotometer wherein the infrared band is at least 9-10 microns toobtain a graphic plot of attenuation v wavelength at selected timeintervals or as a supplement to the alarm arrangement to identify thespecific contaminant that triggered the alarm. The toxicorganophosphorus substances have absorption characteristics which are asindividual as fingerprints.

Crystals of materials other than germanium for multiple internalreflection spectroscopy are known in the art and may be used inpracticing this invention. However germanium crystal is advantageous forfrustrated multiple internal reflection spectroscopy studies of airpollutants because the platinum coating can be wiped off the germaniumeasily and cleanly and the crystal can be recoated either to anotherthickness, or in a different degree of vacuum or with a differentmaterial or any combination of the three Thickness of the coating ismeasured by X-ray fluorescence or by comparing stopping power againstthat ofa reference standard. FIG. 7 shows the effect of thickness ofplatinum film on cell response. Response for platinum is best at about70 millimicrons. With lesser thickness, the film-is less adsorptive.With increased thickness, less of internally reflected infrared energypenetrates to the absorbed substance and, in effect, does not see theabsorbed substance as well.

While other noble metals can be coated on the crystal to obtain the typeof effect described, platinum was found to be superior for the purpose.The crystal base and the opposite parallel surface are coated with theplatinum in not too high a vacuum, i.e. l millimicrons of mercury inorder that the coating have a degree of surface porosity for goodabsorptiveness. An optimum mirror finish obtained by coating in optimumvacuum has poor absorptiveness.

While the crystal shown is in the shape of a parallelogram, in crosssection, the crystal may just as well be in the shape of an isoscelestrapezoid in cross section; the mirror optics is modified accordingly.Though a 45 angle between the sloping sides and the base provides goodresults in using platinum coated germanium, the only limitation on thesize of this angle is that it differ from 90 sufficiently for the angleof incidence to the base or parallel surface exceed the critical angle.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

We claim:

1. A method of using a thin germanium crystal prism having a pair ofopposed parallel surfaces spaced by the thinness dimension and furtherhaving a pair of opposed slant sides at angles suitable for multipleinternal reflection, for monitoring the atmosphere for a particularcontaminant, said method comprising the steps of coating the opposedsurfaces of the prism with platinum by vacuum deposition in a vacuum onthe order of IO millimicrons of mercury to a coating thickness of aboutmillimicrons,

confining a small air-space volume against a selected portion of each ofthe opposed platinum-coated surfaces,

continuously sampling the atmosphere and flowing the sampling throughboth confined air-space volumes and across the platinum coated surfaceportions bordering the volumes,

directing infrared energy of predetermined bandwidth normal to a slantside of the prism for multiple internal reflection therein, and

monitoring the infrared energy existing from the other slant side of theprism for attenuation that characterizes the contaminant.

